The Milky Way is home to hundreds of billions of stars, and many more planets. Some come in sets, as in our own solar system. But not every planet orbits a star.
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Some planets actually wander the galaxy alone, untethered. They have no days or nights, and they exist in perpetual darkness. In a kitschy NASA collection of travel posters for destinations beyond Earth, one of these cold worlds is advertised with the motto: “Visit the planet with no star, where the nightlife never ends.”
Astronomers call these worlds free-floating, or rogue, planets. They are mysterious objects, and a small group of researchers around the world is dedicated to studying them. Of the thousands of planets that scientists have detected beyond our solar system so far, only about a dozen are sunless and coasting on their own, somewhere between us and the center of the Milky Way. At least, astronomers think they are. “We are sure that these objects are planets,” Przemek Mroz, an astronomer at Caltech, told me. “We are not fully sure whether these objects are free-floating or not.”
Mroz has spent perhaps as much time thinking about these strange objects as anyone on Earth. He and his team just announced another finding—the smallest known rogue planet—today. The object is between the masses of Earth and Mars, a blip in interstellar space so relatively tiny that it might seem insignificant. But according to scientists’ best theories about the way planetary systems arise all across the universe, rogue worlds should exist.